A switching element such as a thin film transistor (TFT) is widely used for a picture element switch for a display device such as a liquid crystal display or an EL display. Recently, a case is increasing that a driver circuit for a picture element array is also formed by a TFT on the same substrate.
Generally such a TFT is formed on a glass substrate using an amorphous or polycrystalline silicon. However, there was a problem that it needs increasing cost to fabricate a display device using such silicon materials of a large area because a chemical vapor deposition (CVD) device used for fabricating such a TFT made of silicon is rather expensive. In addition, there was a problem that a substrate was limited on a material, such that a light resin substrate could not be used, because the process to form an amorphous or polycrystalline silicon film is performed at a very high temperature.
To solve such problems, a TFT using a carbon nanotube (CNT) as a semiconductor material is proposed. The practical application of such a TFT using a CNT dispersion film is expected because of merits that a CNT dispersion film is generally formed by a wet process such as an application method and the process can realize a large dispersion film in area at low cost, and selection of a material for a substrate is less restricted because the process temperature is low.
Recently many reports relating to a TFT using a CNT dispersion film are published such as Non-Patent Documents 1 to 4. Also Patent Documents 1 to 3, for example, disclose techniques using a carbon nanotube. The method for preparing a carbon nanotube structure disclosed in Patent Document 1 is “a method including a purification step of a carbon nanotube for preparing a carbon nanotube structure, in which the purification step is to carboxylate a carbon nanotube and react the carbon nanotube with a compound having at least two functional groups that react with a carboxylic group”.
According to Patent Document 1, a purification yield can be remarkably increased because carbon nanotubes are combined to increase the weight (or size) so as to make it easy to recover them and improve the amount of recovered carbon nanotube. It is also described that a carbon nanotube structure can be achieved, in which weight-increased or chemically modified carbon nanotubes are precisely arranged.
The method for forming a pattern of nano-carbon materials disclosed in Patent Document 2 is “a method for forming a pattern of nano-carbon materials layer including a step of forming a nano-carbon materials layer on a substrate, a step of forming a desired pattern of a first metal layer composed of at least a metal selected from the group consisting of zinc, tin, indium, aluminum and titanium on the nano-carbon materials layer, and a step of etching the nano-carbon materials layer by oxygen plasma with a mask of the first metal layer as a positive pattern”.
According to Patent Document 2, it is described that a desired pattern of nano-carbon materials can be formed without deterioration or damage of the nano-carbon materials and thus it is possible to provide high performance semiconductor devices such as an electromagnetic field effect transistor or FED by applying nano-carbon materials.
A thin film transistor disclosed in Patent Document 3 is composed of “an insulating substrate having a uniform thickness and a first main surface and a second main surface both being flat, a gate electrode formed on the first main surface of the insulating substrate, a channel layer formed on the second main surface of the insulating substrate and structured by an organic semiconductor, carbon nanotube, or an organic dispersion material containing at least carbon nanotube, and a source electrode and a drain electrode formed so as to position on both sides of the gate electrode and on the channel layer or between the channel layer and the insulating substrate”.
According to Patent Document 3, it is described that a thin film transistor having stable characteristics can be realized because the insulating substrate as a gate insulating layer has an even thickness and both of the main surfaces are flat, and thus capacity components are uniform in the surface of the substrate. Also it is described that a reliable thin film transistor having improved characteristics of migration degree of carrier in the channel layer and an ON/OFF ratio of current can be achieved because the insulating substrate serves for a gate insulating layer and the channel layer is formed on the second main flat surface that is opposite to the first main surface on which the gate electrode is formed, and thus an evenness of the film thickness of the channel layer is improved. In addition, it is described that the cost can be reduced due to decreased structural elements and fabrication steps because the insulating substrate serves also as a gate insulating layer.    [Patent Document 1]
Japanese Patent Kokai Publication No. JP-2005-125428A    [Patent Document 2]
Japanese Patent Kokai Publication No. JP-2005-347378A    [Patent Document 3]
Japanese Patent Kokai Publication No. JP-2006-73774A    [Non-Patent Document 1] E. S. Snow, J. P. Novak, P. M. Campbell, D. Park, Applied Physics Letters, vol. 82, p. 2145, 2003.    [Non-Patent Document 2] E. Artukovic, M. Kaempgen, D. S. Hecht, S. Roth, G. Grner, Nano Letters, vol. 5, p. 757, 2005.    [Non-Patent Document 3] S.-H. Hur, O. O. Park, J. A. Rogers, Applied Physics Letters, vol. 86, p. 243502, 2005.    [Non-Patent Document 4] T. Takenobu, T. Takahashi, T. Kanbara, K. Tsukagoshi, Y. Aoyagi, Y. Iwasa, Applied Physics Letters, vol. 88, p. 33511, 2006.